1. Field of the Invention
The present invention relates to an optical pickup device, and more specifically, is suitable for use in an optical pickup device that records/reproduces information on/from a recording medium in which a plurality of recording layers is laminated.
2. Disclosure of Related Art
In recent years, optical discs have been increasing in capacity. High-capacity optical discs can be realized by arranging a plurality of recording layers in one disc. For example, there are commercialized multiplayer-type disc such as digital versatile discs (DVDs) and Blu-ray discs with two recording layers on one side. Providing three or more recording layers would further increase optical discs in capacity.
However, when a plurality of recording layers is provided in a disc, light reflected from recording layer(s) other than a recording layer as a reproduction target enters into a photodetector to thereby deteriorate reproduction signals. This is a so-called interlayer crosstalk problem due to stray light. Therefore, disposing a plurality of recording layers in one disc requires an arrangement for smoothly suppressing and eliminating stray light.
As a method to eliminate stray light, a pinhole is placed in front of a photodetector, for example. In another stray light eliminating method, front and rear light shield plates are arranged in front of a photodetector. Further, there is another method in which a polarization optical element is employed.
However, under the method with a pinhole in front of a photodetector, both reflected light from a recording layer as a reproduction target and stray light pass through the pinhole and reach the photodetector. In this case, if a space between the recording layers is small, a larger amount of stray light passes through the pinhole and thus interlayer crosstalk cannot be properly eliminated. According to the method with front and rear light shield plates, half of reflected light from a recording layer as a reproduction target is blocked, which reduces usage efficiency of the reflected light to about 50%. Further, the method with a polarization optical element requires a large number of optical components for stray light elimination, thereby resulting in cost increase.